Multi-carrier modulation techniques, such as orthogonal frequency division multiplexing (OFDM), have become widely used in high-speed wireless communications. By partitioning a wideband fading channel into narrowband channels, OFDM is able to mitigate the detrimental transmission effects such as multipath fading. However, in the time domain OFDM signals suffer from large envelope variations, which are often characterized by the peak-to-average ratio (PAR).
These variations are the result of the cancellation and accumulation of waveform symbols transmitted in different frequency channels. When the phase of all waveform symbols align, a large peak in the combined waveform is created. Large peaks complicate the transmission and reception of the waveform, because transistors have a limited range of operating voltage. If the amplitude of a received waveform is greater than the operating voltage of a transistor, the crest of the waveform is clipped. To avoid clipping high PAR signals, transmission amplifiers must operate at very low power efficiencies. Individual symbols in each channel frequency are transmitted with smaller amplitudes. However, this results in lower signal-to-noise ratios. Smaller individual peaks in each frequency become harder to distinguish from noise, and reflections present on the channel and may corrupt the symbols received by the receiving antenna.
The present invention may address one or more of the above issues.